During the process of drilling for hydrocarbons, there is often the need to set a cementitious material plug in an open hole to allow the process of sidetracking and drilling of a new well bore. It is possible to drill multilateral wells into different parts of a reservoir from a single wellbore by a method known as directional drilling. Many directional wells are drilled to reach reservoirs inaccessible from a point directly above because of surface obstacles and/or geologic obstruction. Wellbore sidetrack drilling operations with hard cementitious material plugs are well known in the art. Wellbore sidetrack drilling comprises placing a cementitious material plug in a borehole and allowing the cementitious material to develop high compressive strength. The hardened cementitious material plug may deflect a drill bit away from the current borehole, starting another open hole section. Conventional cementitious material formulations for sidetrack kickoffs usually fail when the ROP (Rate of Penetration) for the cementitious material plugs is much higher than the ROP in the surrounding formation. Sidetracking failure, in building up a kickoff angle, results in operational delay and cost overrun.
Generally, a length of approximately 20 m to 30 m of good cementitious material is required in a well bore to form a plug in order to perform a successful side track. Poor cementitious material can lead to failure to create successful sidetracks, requiring further work placing cementitious material plugs or other remedial work that is expensive to rig operators. In sidetrack operations, an average of 2.4 attempts per sidetrack, with 24 hours with each attempt, has been reported and experienced in the field. Failures in sidetrack cementitious material plugs can occur because of plug slippage, insufficient plug curing time, insufficient slurry volume, slurry composition, slurry losses while extracting equipment, and/or poor mud removal (e.g. due to using an unsuitable spacer).
Cementitious material plugs are placed in oil and gas wells for various reasons other than sidetracking, including well abandonment, squeezing (e.g. where a cementitious material slurry is injected into an isolated zone) and zone isolation. Cementitious material plug placement may be used to block off a hole, for subsequent re-drilling through the cementitious material plug. This may be the case if curing down hole mud losses, or exceptionally if stability of the hole walls is low or if there is a risk of hole collapse.
There can be great difficulty in placing good cementitious material in sections of a hole if there are large washouts (e.g. where the diameter of the hole suddenly increases, forming a cavern type region, due to for instance partial hole collapse). Sometimes washouts can be up to twice the diameter of a drilled hole. In rare cases, washouts can be more than twice the diameter of a drilled hole. The current procedure is to pump excess cementitious material to fill an over-gauge wellbore. This is not effective in all situations as the velocity of the pumped cementitious material in an annulus between a down hole assembly and the interior surface of a well bore (i.e. the ‘annular velocity’ of the pumped cementitious material) is so low that mixture of the cementitious material with drilling mud can occur, which contaminates the cementitious material preventing it from gaining full strength; i.e. contamination reduces the strength of the cementitious material.
Density, rheology and hole angle are major factors affecting plug success. While the Boycott effect (i.e. that sediment settles faster in an inclined hole, and slide as a mass to the lower side of an inclined borehole) and an extrusion effect (e.g. the flow of liquid slurry out of a delivery device) are predominant in inclined wellbores, a spiralling or “roping” effect controls slurry movement in vertical wellbores. Current understanding of down hole flow mechanics is unable to explain all of the unsuccessful attempts at forming cementitious material plugs. For example, plug tops have varied with no apparent pattern, and some plugs have drilled softer than expected. Although large excess volumes of cementitious material are commonly used to improve the chances of success, in such jobs, these volumes can pose other problems. For example, the plug top may be extremely high, which would result in excessive rig time for drilling new formation, and larger volumes of cementitious material-contaminated mud will likely result. Concerns are also commonly raised about the capability of successfully pulling a work string out of the resulting long slurry columns before the onset of cementitious material gelation and/or hydration.
Long-term plug stability based on accepted industry standards is highly debatable. Abandonment plugs fail, despite the fact that they were thought to have been properly set according to all regulatory guidelines. Factors affecting plug stability include, but are not limited to only: wellbore angle including vertical, deviated and horizontal; hole size; spotting fluid and wellbore fluid rheologies and densities; and work string and/or hole diameter annulus.
In conventional wellbore drilling, a first section of a hole may be drilled and a casing (for instance, made of metal) may then be run into that first section, which may be secured in place by cementitious material. A second section of hole may be drilled as a continuation of the first section. The second section is often of a smaller diameter, due to the drill bit being limited in size by the internal diameter of the casing present in the first section. That is, at each stage, the diameter of hole is limited by the size of tool that can be run through the internal diameter of the previous stage's casing. Wellbores can reach around 10 km in length. However, it is known to use an underreaming tool that can make the second section have a larger diameter than the internal diameter of the casing in the first section. In this case, the underreaming tool may be run through the metal casing of the first section in a collapsed state. An example of such an underreaming tool/underreamer is the custom built Underreamer “ADT” model produced by Adriatech S.r.l. of Pescara, Italy. Therefore, in practice, the diameter of hole to be filled with cementitious material may be larger or smaller, or the same size, as a section of hole through which a cementitious materialing assembly must be run.
US2011/0162844A1 discloses a bottomhole assembly for placing a cementitious material plug in a wellbore, comprising an elongate support structure having annular seals that slide against the internal surface of a hole or hole casing. The seals are provided at opposing ends of the support structure, and cementitious material is pumped into the annular region between the seals. The support structure is left in the well after the cementitious material has cured.
U.S. Pat. No. 6,269,878 describes a bottomhole assembly for plugging a wellbore, comprising a runner configured for connection to a drill pipe and for delivering cementitious material down hole, and a packer for anchoring the cementitious material in the wellbore, the packer being connected to the exterior of one end of the runner and comprises a rigid structural part supporting an expandable cover. Cementitious material is pumped into the expandable cover, which remains connected to the rigid structural part. The rigid structural part may be disconnected from the drill pipe, and is left in the well after the cementitious material has cured.
Poly Diamond Crystalline (PDC) drill bits are generally favoured because they produce higher drilling rates, are longer lasting for conventional drilling (thus saving extraction of a drill pipe to replace a worn bit), and are less likely to break down hole because they have no moving parts. However, steel is not readily drillable with a PDC drill bit. Steel can be drilled with mill tooth bits and junk bits, but PDC bits are particularly susceptible to damage; i.e. chipping of the cutters and so reduce bit performance when drilling ahead. Accordingly, it is desirable to have a means for creating a cement plug that does not contain steel components therein.